In the past ten years, compact discs (CDs) have become a dominant format for distribution of music recordings, due to their compact size and high-quality digital reproduction of music.
Conventional, mass-produced CDs store data as a pattern of pits and lands, representing digital "1"'s and "0"'s, following spiral tracks on a reflective surface of the disc. To read the CD, a read laser scans the reflective surface, detects the pits and lands by changes in reflection from the disc surface, and decodes the digital information. CDs are mass-produced by first patterning a photoresistive surface of a glassmaster (using a laser beam) with a pattern of pits and lands indicating the data recorded on the disc. This pattern is then embossed or stamped into a reflective material such as polycarbonate, which is then encased in a protective plastic coating.
Although audio reproduction was the primary motivation for development of the CD, because CDs store vast volumes of digital information (approximately 600 million eight-bit bytes), and because of cost reductions resulting from the popularity of audio CDs, the CD has recently become a preferred form for storing data for a computer in the form of read only memory (CD-ROM). As a result, there are now a number of formats in which CD information is stored.
The format in which audio information is stored on a CD is known as the "redbook" standard. Under redbook, digital data on a CD is organized into indexed tracks. The digital samples for left and right audio channels are interleaved with error correcting codes and subcode data. Throughout the disc, the interleaved subcode information identifies the current position, in minutes and seconds, both with respect to the current track and with respect to the entire disc. The subcode also includes information indicating the beginning and end of tracks. Furthermore, in a lead-in section preceding the first track (encoded with audio silence), the interleaved subcode information includes a table of contents identifying the position of each track in the remainder of the disc. And, in a lead-out section following the final track on the disc, the interleaved subcode information includes special encoding indicating the end of the program material.
The so-called "yellowbook" standard is typically used as a format for a CD-ROM. The "yellowbook" format is similar to the "redbook" format in many respects, including the use of data organized into tracks, interleaved with error correction and subcode information. The interleaved subcode identifies, in minutes and seconds, the current position with respect to the track and the disc as a whole, as well as information indicating the beginning and end of a track. Furthermore, the disc includes a lead-in section including (in the subcode) a table of contents, and a lead-out section indicating (in the subcode) the end of the information encoded on the disc. The primary differences between the redbook and yellowbook formats (other than the nature of the stored data) are the inclusion, in yellowbook discs, of extra error correction information, and a marker in the subcode in the lead-in indicating that the disc is a CD-ROM rather than an audio CD.
A relatively new extension of yellowbook, known as CD-XA, was developed for CD-ROMs which carry multiple different kinds of data, for example, audio, video, and text data. In a CD-XA disc, each track contains interleaved video, audio, text, and other types of data. A CD-XA compatible player can combine the audio, video, and text in real-time into a single multimedia output.
Furthermore, a so-called multi-session format was developed for use with user-recordable CDs. Recordable CDs begin as a blank disc with a normally-reflective, heat sensitive surface. These blank discs are written by heating the surface of the disc with a high power laser beam, causing heat-induced chemical change in the reflectivity of the surface. The resulting pattern of reflective and nonreflective areas can be read by the laser in a CD player. It is not possible to reverse the chemical changes which modify the surface reflectivity of the disc and thereby stores information. Thus, recordable CDs are a write-once (CD-WO) media, i.e., they can only be recorded once, and cannot be erased or re-recorded.
The multi-session format was created to overcome one difficulty arising from CD-WO technology. Specifically, the CD formats noted above require encoding a disc with a table of contents indicating the locations of tracks on the disc, and further require a lead-out section indicating the end of information encoded on the disc. This information will necessarily change if the data on the disc changes, i.e., if a track is added to the disc after initial recording. However, as noted above, the data on a recordable CD-WO cannot be changed after initial recording. Thus, using the formats discussed above, all of the information on a recordable CD would have to be recorded at one time, rather than incrementally.
To overcome this difficulty, the multisession format records multiple, separate tables of contents in multiple, separate sections of the disc. Specifically, the disc includes multiple "sessions", each having a number of tracks, a lead-in section including subcode information describing a table of contents for tracks in the session, and a lead-out section with subcode information indicating the end of the session. Because there are separate tables of contents for separate sessions on the disc, data can be recorded on the disc incrementally; once a session has been completed (and a corresponding table of contents and lead-out created), that session can be played from the disc by a multisession player notwithstanding whether other sessions have been completed. Furthermore, data can be added to a partially recorded disc by simply adding a new session (or adding tracks to an existing, partially completed session).
One difficulty arising out of this multitude of formats is cross-compatibility of formats with players. For example, if a user places a yellowbook or CD-XA disc in a pre-1986 CD player (i.e., a player designed prior to the emergence of CD-ROM and designed only to play audio CDs), the player will not recognize the codes identifying the disc as a CD-ROM disc, and will attempt to play the tracks on the disc as music. Although the CD-ROM tracks which actually contain music or audio information in redbook format will play properly, CD-ROM tracks containing other data will play as broadband noise, grating the user and potentially damaging the users' stereo equipment.
To avoid the scenario described in the preceding paragraph, since about 1986 manufacturers of audio CD players have typically included circuitry which recognizes the unique lead-in coding identifying a CD-ROM track; typically, this circuitry prevents the player from producing an output during CD-ROM tracks. Thus, although owners of post-1986 players are spared listening to noise generated by CD-ROM tracks, these tracks will not be skipped; rather they will play as long periods of silence.
It has been suggested to include non-audio data on musical discs as a supplement to, the liner notes typically included with a musical CD. The CD-ROM data could include lyrics, still video, etc. The original redbook standard sets aside some space in the interleaved subcode for such a purpose; however, few discs have made use of this subcode space. Efforts to use subcode space have been frustrated by the limited space available in the subcode, and the low data rate at which subcode information can be retrieved (both are insufficient, for example, to store real-time video).
As noted above, yellowbook or CD-XA formatted discs have been encoded with sound and accompanying video or text. However, for the reasons noted above, post-1986 audio CD players are designed to play silence during CD-ROM tracks on yellowbook or CD-XA discs, and pre-1986 audio CD players play the CD-ROM tracks as noise. Therefore, discs formatted with audio, video and data can only be satisfactorily played using a computer and a yellowbook or CD-XA compatible player. As a result, to date only a small number of such discs have been produced.